Season changes in breeding efficiency among colonies of rhesus monkeys remain a controversial issue with little pertinent scientific data available. Effects of season on hormonal parameters important in male fertility gonadal secretions are involved. Measurements of Follicle Stimulating Hormone, Leutoining Hormone, Testosterine, and Androsternedione in peripheral serum will be assayed for five consecutive days every month in ten adult breeder male monkeys. Correlations between hormone levels, breeding efficiency, and season will be determined.